Japanese Unexamined Patent Publication (A) No. 11-270399 describes a device for judging the property of the actually used fuel in an internal combustion engine. In this device, a difference between a time for a specific point of a crankshaft to pass a predetermined set crank angle range (specifically 30°) right after an air-fuel mixture is ignited and a time for the specific point of the crankshaft to pass through the same set crank angle range right after the air-fuel mixture is next ignited is calculated and it is judged that heavy fuel is being used when this difference exceeds a reference value.
However, in the above device, the crank angle range over which the time for passage by the specific point of the crankshaft is measured is small, so the difference in times for passage of the specific point of the crankshaft is small. Therefore, the precision of judgment of whether this difference exceeds the reference value is low. As a result, even if controlling the ignition timing for an air-fuel mixture based on the fuel property judged by this device, improvement of the fuel consumption and reduction of the exhaust emission may not be achieved as desired.